[PATCH] kernel/fork: Increase minimum number of allowed threads

[PATCH] kernel/fork: Increase minimum number of allowed threads

[Hauke Mehrtens]

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

A modern Linux system creates much more than 20 threads at bootup.
When I booted up OpenWrt in qemu the system sometimes failed to boot up
when it wanted to create the 419th thread. The VM had 128MB RAM and the
calculation in set_max_threads() calculated that max_threads should be
set to 419. When the system booted up it tried to notify the user space
about every device it created because CONFIG_UEVENT_HELPER was set and
used. I counted 1299 calles to call_usermodehelper_setup(), all of
them try to create a new thread and call the userspace hotplug script in
it.

This fixes bootup of Linux on systems with low memory.

I saw the problem with qemu 10.0.2 using these commands:
qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic

Cc: stable@vger.kernel.org
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 7966c9a1c163..388299525f3c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -115,7 +115,7 @@
 /*
  * Minimum number of threads to boot the kernel
  */
-#define MIN_THREADS 20
+#define MIN_THREADS 600
 
 /*
  * Maximum number of threads
-- 
2.50.1

Re: [PATCH] kernel/fork: Increase minimum number of allowed threads

[Hauke Mehrtens]

On 7/12/25 01:03, Hauke Mehrtens wrote:
> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Sorry this has the wrong from tag, will send a new patch.

Hauke>
> A modern Linux system creates much more than 20 threads at bootup.
> When I booted up OpenWrt in qemu the system sometimes failed to boot up
> when it wanted to create the 419th thread. The VM had 128MB RAM and the
> calculation in set_max_threads() calculated that max_threads should be
> set to 419. When the system booted up it tried to notify the user space
> about every device it created because CONFIG_UEVENT_HELPER was set and
> used. I counted 1299 calles to call_usermodehelper_setup(), all of
> them try to create a new thread and call the userspace hotplug script in
> it.
> 
> This fixes bootup of Linux on systems with low memory.
> 
> I saw the problem with qemu 10.0.2 using these commands:
> qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic
> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
> ---
>   kernel/fork.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7966c9a1c163..388299525f3c 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -115,7 +115,7 @@
>   /*
>    * Minimum number of threads to boot the kernel
>    */
> -#define MIN_THREADS 20
> +#define MIN_THREADS 600
>   
>   /*
>    * Maximum number of threads

Re: [PATCH] kernel/fork: Increase minimum number of allowed threads

[David Laight]

On Sat, 12 Jul 2025 01:06:07 +0200
Hauke Mehrtens <hauke@hauke-m.de> wrote:

> On 7/12/25 01:03, Hauke Mehrtens wrote:
> > From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>  
> 
> Sorry this has the wrong from tag, will send a new patch.
> 
> Hauke>
> > A modern Linux system creates much more than 20 threads at bootup.
> > When I booted up OpenWrt in qemu the system sometimes failed to boot up
> > when it wanted to create the 419th thread. The VM had 128MB RAM and the
> > calculation in set_max_threads() calculated that max_threads should be
> > set to 419. When the system booted up it tried to notify the user space
> > about every device it created because CONFIG_UEVENT_HELPER was set and
> > used. I counted 1299 calles to call_usermodehelper_setup(), all of
> > them try to create a new thread and call the userspace hotplug script in
> > it.
> > 
> > This fixes bootup of Linux on systems with low memory.

I bet it doesn't - it is likely to fail somewhere else instead.
While 20 is probably too low, the real issue seems to be that
the hotplug notifications need rate limiting.

	David

> > 
> > I saw the problem with qemu 10.0.2 using these commands:
> > qemu-system-aarch64 -machine virt -cpu cortex-a57 -nographic
> > 
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
> > ---
> >   kernel/fork.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 7966c9a1c163..388299525f3c 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -115,7 +115,7 @@
> >   /*
> >    * Minimum number of threads to boot the kernel
> >    */
> > -#define MIN_THREADS 20
> > +#define MIN_THREADS 600
> >   
> >   /*
> >    * Maximum number of threads  
> 
>